This invention relates to an electrically conductive elastomeric foam and a method of making the same. More particularly, this invention relates to a new electrically conductive elastomeric foam, incorporating electrically conductive fibers, which exhibits improved conductivity over other electrically conductive foam materials, and which retains good mechanical and physical properties such as resiliency, compression set resistance, low durometer hardness, small uniform cell structure, strength and wear resistance.
The field of soling materials in electrostatic dissipative (ESD) footwear and the field of rollers, belts and other members used in contact with photoreceptor members in electrophotography are particularly pertinent, so these fields will be discussed for purposes of illustration of the features, utility and advantages of this invention. However, it is to be understood that this invention is not limited to either field; rather this invention is generally well suited for any application utilizing electrically conductive elastomeric foam (particularly polyurethane foam) and this invention is especially well suited for any application in which ESD and EMI/RFI shielding is desired.
Accumulation of static electrical charge in certain working environments has long been recognized as undesirable. Many widely used synthetic materials, for example polyurethane and polyvinyl chloride are electrical insulators and articles of these materials may thus accumulate a substantial static electric charge.
Elastomeric foams, including polyurethane foams, are widely used in the footwear industry to prepare electrostatic inner soles, outer soles and insole inserts. Electrically conductive materials are used to prevent electrical charges from building up on the worker's body. These electrostatic charges can pose a serious threat of injury if the air contains combustible gases or flammable liquid vapors. When a worker touches a grounded metal object, the buildup of electrostatic charge may cause a spark, which in turn may cause airborne combustibles to explode. In addition, electrically conductive footwear is useful for workers who handle electronic equipment, which is easily damaged by static discharges.
Electrically conductive footwear keeps electrostatic charges from accumulating by providing a conductive path of relatively low electrical resistance from the foot to the floor. Use of electrically conductive outersoles and insoles maintain a conductive path, allowing electrical charges to be transferred from the user's foot to the conductive shoe. Patents which describe various types of electrically conductive sole and sole inserts include: U.S. Pat. No. 4,861,805 issued to Saavedra et al (directed to a shoe sole) and U.S. Pat. No. 5,319,867 issued to Weber (directed to a shoe insole).
In the conventional electrophotographic process, toner is metered from a toner cartridge onto a photoreceptor on which a latent electrostatic image has been formed. The toner is then transferred to and fixed on paper or other substrate or print media. Each step of the electrophotographic process requires precise control of the amount of electrostatic charge present. The steps performed around the central photoreceptor (PC) drum are drum charging, exposing and developing, transfer of toner from the drum to the print medium (usually paper), and drum cleaning. Each of these steps, except exposing, can use electrically conductive elastomeric rollers, belts or other members. The electrical conductivity requirements depend on the function performed and the machine design. The conductivity range can require elastomeric materials with volume resistivities ranging from 10 to the power of 3 to 10 to the power of 10 ohm-cm. Several of these functions including charging, transferring and cleaning can be improved through the use of conductive elastomeric foam members. Elastomeric foam provides higher compliance (lower durometer hardness, or lower compression force deflection) which provides a greater footprint against the PC drum with lower pressure and therefore lower abrasive wear compared to non-foamed (solid) elastomers. Low modulus (high compliance) foam rolls also have utility as toner supply rolls, which are part of the PC drum developing system. An application for a patent describing an electrically conductive polyurethane foam for use in rollers in conventional electrophotographic process is Japanese Application No. HEI 2-262715 262,715/1990!.
Accumulation of electrical charge in certain equipment environments has also long been recognized as undesirable. Accumulation of electrical charge on equipment components may attract dust, which may adversely affect the quality of manufactured products; electrical discharge may disturb the performance of electromagnetic machines, such as computers, in the vicinity of the discharge; fire or explosion may result from such discharge in environments used to store combustible materials, or in grain elevators. It has long been known that grounded articles of electrically conductive material will dissipate electrical charge.
As is apparent from the foregoing discussion, there presently exists a need for electrically conductive elastomeric foams, particularly electrically conductive polyurethane foams. Currently, one method of rendering elastomeric foams electrically conductive is by the incorporation of ionic compounds therein. For example, U.S. Pat. No. 4,861,805 discloses that a polyurethane shoe insole or outersole may be rendered electrically conductive by incorporating a non-volatile ionizable metal salt therein.
However, the use of ionic compounds in elastomeric foams suffers from certain limitations and drawbacks. For example, the lowest electrical resistance (highest conductivity) achievable using ionic compounds in elastomeric foams is about 1.times.10.sup.8 ohm-cm. In addition, conductivity for such foams with ionic conduction is very sensitive to temperature and humidity. In addition, because conductivity is affected by ion mobility, conductivity changes over time due to ion depletion.
These disadvantages can be at least partly overcome by the incorporation of conductive fillers. U.S. Pat. No. 4,505,973 discloses a rigid polyurethane foam rendered electrically conductive by incorporating therein various carbon blacks. Japanese Application No. HEI 2-262715 262,715/1990! describes an electrically conductive polyurethane foam incorporating a carbon micropowder of particle size smaller than 100 .mu./m. However, the use of conductive fillers in elastomeric foams suffers from the need to use high filler loadings. High filler loadings adversely affect processing and the mechanical properties of the finished foam, especially polyurethane foams, and often makes the finished foam relatively expensive.
Conductive fibers have been added into insulating polymers, such as polyurethane, to render them electrically conductive with lower additive concentration than would be required with particulate conductive filler, according to U.S. Pat. No. 4,228,194. This patent discloses conductive fibers coated with silicone oil to effect high conductivity at low fiber loadings. However, silicone oil has a destabilizing effect on foam structure in the processing of elastomeric foams. This approach is not expected to be applicable to foamed elastomers with good mechanical properties and fine (small) uniform cell structure.
Fibers, including electrically conductive fibers, can be incorporated into elastomeric silicone foam according to U.S. Pat. No. 4,572,917. However, these foams do not possess the high level of mechanical properties, such as strength, toughness and abrasion resistance, necessary for many applications including the ESD footwear inner and outer sole materials and the electrophotographic rollers and other members mentioned previously.